The El Niño Southern Oscillation

The El Nino Southern Oscillation, or ENSO, describes water temperatures in the eastern tropical Pacific Ocean and the linkages to weather and climate patterns around the world. ENSO tends to be the best predictor of our weather (in North Carolina) on a seasonal timescale.

Typical Annual Pattern

For most of the year in the tropics, the trade winds push the warm surface waters of the equatorial Pacific Ocean westward, allowing the upwelling of cold, deep ocean waters in the eastern Tropical Pacific. As the seasons shift, these trade winds weaken, and warm water is able to extend farther eastward — this typically happens around December. In this normal pattern (which is called the Walker Circulation), warmer water piles up in the western equatorial Pacific Ocean — in fact, the elevation of the ocean off the east coast of Australia is as much as 40 cm higher than the elevation of the ocean surface off the west coast of South America! The warm surface water imparts this characteristic to the air above it: the air becomes warmer, less dense, and rises. For this reason, in a typical year we would see more rising air, clouds, and storms over the western equatorial Pacific. In contrast, in the eastern equatorial Pacific, the cool waters lead to cooler and denser air, which sinks; clouds and precipitation are unlikely, and this is one of the reasons the Atacama desert is on the west coast of South America. This typical pattern, combined with the Earth’s rotation, leads to prevailing easterlies (the Trade Winds) in the equatorial Pacific Ocean. Sometimes this typical pattern intensifies or even completely reverses; when this happens, we experience different phases of the El Niño Southern Oscillation (ENSO) known more commonly as El Niño and La Niña. Read the sections below to learn more about each of these climate patterns and what they mean for our weather in North Carolina.

Note the temperature shift in the equatorial Pacific from warmer (deep red) to cooler (blue).
Image from NASA